2 Corrosion Final

8/13/2019 2 Corrosion Final

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CorrosionTommy Lundin



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•Process• Corrosion



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Introduction

• Oxidation (and thus corrosion) is the aim of the nature to reachequilibrium

• Therefore, all boilers will corrode

• The only thing we can influence is the rate

•This summary deals only with gas side corrosion processes

• Steam side corrosion is not considered in this summary, but it shouldnot be totally neglected

• The flue gas moisture content may have a significant effect on thecorrosion process and corrosion rate of alloys



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• High temperature oxidation

• Hot corrosion

• Active oxidation

• Smelt induced corrosion

• Sulfidation

• Acidic sulfates

Corrosion types in recovery boilers



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Lower furnace:• Bottom tubes

• Wall tubes

• Air ports

Location of corrosion in recovery boilers

Upper furnace:• Superheaters

• Superheater ties

Second pass:• Boiler bank

• Economizers



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High temperature oxidation

• One of the main reasons for long-term tube wastage in boilers• Limits the use of low-alloyed steels at high steam temperatures

• Present in most corrosion processes

• Increases corrosion rate as a secondary process in connection with

the primary mechanism• Can not be totally avoided, but can be controlled by using high

chromium (Cr) alloys



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0.0

5.0

10.0

15.0

20.0

25.0

30.0

100 280 460 640 820 1 000

Temperature (C)

P e n e t r a t i o n ( m m )

Carbon steel AISI 304 AISI 316

AISI 310 AISI 347

High temperature oxidation

• Oxidation typically occurs whenmetals are exposed totemperatures above 300 C

• Wüstite (FeO) formation above570 C increases the oxidation

rate of low-alloyed steels• Chromium (Cr) alloying

increases oxidation resistanceof the alloys

10 000 h, 5% O2



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Hot corrosion

• Main contributors to the FMT (T0) are chlorine (Cl) and potassium (K)• High amount of carbonates (CO3) i.e. carryover lowers the FMT of

the deposits

• Even small amount of sulfides (0.1 wt.-%) in the deposits lowers theFMT about 50 C

41 Na2SO4 - 59 K2SO4 831

26 KCl - 74 K2SO4 689

50 NaCl - 50 KCl 656

48 NaCl - 52 Na2SO4 628

28 KCl - 38 NaCl - 14 K2SO4 - 20 Na2SO4 518

Deposit mixture (mol-%) FMT (°C)



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Hot corrosion

• Main contributors to hot corrosion are chlorine (Cl) and alkalis (Na,K)

• Pure compounds rarely cause problems, but a mixture of compoundsmay produce an extremely low FMT

• Two non-corrosive compounds may form a low melting mixture andmay become highly corrosive

• The main corrosion mechanism in hot corrosion is acidic or basic fluxing

• In practice this mean that the metal or alloy dissolves into the melt



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KVAERNER POWER

Chlorine influence on ash melting behaviour

• Chlorine (Cl) does not affectthe FMT of the deposits

• Increase in Cl content,increases the amount of meltin deposits

0

20

40

60

80

00

500 600 700 800

0,2%

1%

5%

Temperature, °C

M e l t , %

T70

T15

Chlorine, wt-% in ash:



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KVAERNER POWER

0

20

40

60

80

100

500 600 700 800

Potassium influence on ash melting behaviour

• Potassium (K) does notaffect the amount of melt inthe deposits

• Increase in K content,decreases the FMT

0,2%

2%

5%

Temperature, °C

M e l t , %

T70

T15

Potassium, wt-% in ash:



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KVAERNER POWER

0

20

40

60

80

00

500 600 700 800

Sulfide influence on ash melting behaviour

• Sulfide (S2-) does not affectthe amount of melt in thedeposits

• Increase in S2- content,decreases the FMT

0%

0,1%

1%

Temperature, °C

M e l t , %

T70

T15

Sulfide, wt-% in ash:



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Hot corrosion

• Gaseous potassium chloride (KCl) can react with chromium oxidescale, forming potassium chromates:

Cr 2O3(s) + 4 KCl(g) + 2 H2O(g) + 1.5 O2(g) 2 K2CrO4(s) + 4 HCl(g)

Cr 2O3(s) + 4 KCl(g) + 2.5 O2(g) 2 K2CrO4(s) + 2 Cl2(g)

•

Even low concentrations (~5 ppm) of gaseous KCl can be verycorrosive

• The protective properties of the oxide scale are lost

• Gaseous KCl is blamed to be one of the main contributors to thebreakdown of the protecting oxide scale of alloys



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Active oxidation

• Active oxidation is a chlorine-induced corrosion process- Initiated by the release of volatile chlorine from alkali chlorides or by the reaction

of alkali chlorides with the oxide scale

- Chlorine reacts selectively with Fe in the alloy, forming volatile iron chlorides

- Iron chlorides are oxidized, releasing chlorine back to the process

•Threshold temperature 450 °C

- Active oxidation is not present when metal surfaces are below this temperature

• Activated oxidation is present only when potassium chloride (KCl) isavailable



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Active oxidation

• Chlorine acts as a catalyst and may initiate achlorine circulation process in the material

- Released chlorine will react again with the metal- Part of the chlorine will escape from the metal and

the process will eventually cease without anexternal Cl source

• Corrosion by activated oxidation processproceeds along the grain boundaries

- Intergranular oxidation- Present only in high chromium alloys



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KVAERNER POWER

Smelt induced corrosion

• Molten alkali hydroxides are blamed to

be responsible for the primary air port corrosion

• Molten alkali hydroxide (MeOH) mayreact with chromium oxide scale, formingalkali chromates:

Cr 2O3(s) + 4 MeOH(l) + 1.5 O2(g)

2 Me2CrO4(dissolved) + 2 H2O(g)

• Formed alkali chromate dissolves into

the melt and is nonprotective



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Sulfidation

• Occurs when metals are exposed to temperatures above 200 °C ingases containing more than 1 ppm hydrogen sulfide gas (H2S)

• Reducing conditions may be due to low air ratio or high unburntcarbon content

• Gaseous methyl mercaptan (CH3SH) increases corrosion rate attemperatures higher than 320 °C

• The presence of gaseous hydrogen chloride (HCl) increases thecorrosion rate in reducing conditions

• The corrosion rate is related to the formation of metal sulfides, which

contain large amount of defects



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Sulfidation

• Sulfidation rate increases withtemperature

• High steam temperatures requirethe use of AISI 304 composite furnace wall tubing

• Alloy AISI 304 is applicable attemperatures below 350 C

• High nickel (Ni) alloys susceptible for sulfidation

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,70

0,80

0,90

1,00

250 270 290 310 330 350

Temperature (C)

P e n e t r a t i o n ( m m )

Carbon Steel AISI 304 AISI 316

AISI 310 AISI 347

1 000 ppm H2S, 20 000 h



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Prevention of corrosion

•Flue gas inlet temperature into

- Superheater area

- Boiler bank

• Proper mixing in the furnace- Air delivery and staging

- Black liquor spraying and distribution

• Location of the burners

- CNCG burners preferentially below liquor guns

• Lower furnace materials

- High Cr alloy composite tubing

• Arrangement of heat transfer surfaces



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Prevention of corrosion

• High Cr content of alloys increases oxidation resistance- Wüstite formation above 570 °C

- Cr content preferably higher than 20%

- Intergranular oxidation of austenitic steels

• High Ni content increases resistance to chlorine-induced corrosion

- Ni susceptible to sulfidation

• High Cr alloys vulnerable to alkali chromate formation- Favored in high potassium atmospheres



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Summary

• Gaseous HCl is not causing problems in oxidizing conditions, whensteam temperature is below 550 °C

• Alkali chlorides are the main cause for corrosion- Low FMT molten phase corrosion

- Sulfation of chlorides active oxidation

- Cracking of oxide scales enhanced oxidation

• Threshold material temperatures- 450 °C for chlorine-induced corrosion

- 480 °C for hot corrosion

2 Corrosion Final

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